Processes for the selective epoxidation of olefins which contain no allylic hydrogen atoms (non-allylic olefins) or olefins which contain hindered allylic hydrogen atoms are described in U.S. Pat. Nos. 4,897,498, 4,950,773, 5,081,096, 5,138,077 and 5,145,968. U.S. Pat. No. 5,117,012 describes the selective epoxidation of 1,3-butadiene to 3,4-epoxy-1-butene (EpB) by contacting a mixture comprising 1,3-butadiene, oxygen and methane with a supported silver catalyst at elevated temperatures. U.S. Pat. No. 5,362,890 describes an improved process for the selective epoxidation of 1,3-butadiene to 3,4-epoxy-1-butene (EpB) wherein the ballast gas for the reaction is n-butane. U.S. Pat. No. 5,618,954 describes a similar process for the epoxidation of 1,3-butadiene to 3,4-epoxy-1-butene with nitrogen or C.sub.1 -C.sub.4 hydrocarbons (especially methane) as the diluent.
The use of diluent or ballast gases in ethylene epoxidation is described in Canadian Patents 1,286,687 and 2,053,404 and U.S. Pat. Nos. 3,119,837 and 5,057,481. According to these patents, the typical molar composition of feed gases used in such ethylene epoxidation processes comprise up to 30 mole percent ethylene, up to 12 mole percent oxygen, up to 7 mole percent carbon dioxide, up to 5 mole percent ethane with the balance being composed of an additional inert diluent such as nitrogen or methane. U.S. Pat. No. 3,119, 837 teaches that selectivity of ethylene conversion to ethylene oxide can be enhanced by the addition of methane as a diluent. Methane serves as a heat sink, moderating temperature differentials within the reactor, and allows for more isothermal reactor operation. This patent further states that the benefits to selectivity and ease of operation do not extend to other paraffins normally encountered in commercially available ethylene, e.g., ethane and propane. Use of methane also allows an increase in the oxygen:ethylene ratio in the reactor feed gas which increases conversion of ethylene to ethylene oxide.
According to Canadian Patent 1,286,687, other diluents that function as heat sinks include nitrogen, helium, argon, carbon dioxide and lower paraffins such as methane and ethane. However, U.S. Pat. No. 5,057,481 teaches that the use of ethane at concentrations greater than about 5 mole percent results in reduced selectivity in the epoxidation of ethylene to ethylene oxide and lower thermal stability because the chloride concentration on the catalyst surface is lowered. Typical silver catalysts employed in the epoxidation of ethylene contain about 1 to 300 parts by million by weight (ppmw) of Cl on the catalyst surface, both to increase selectivity to ethylene oxide by lowering combustion of ethylene and ethylene oxide to carbon dioxide and water as well as to increase the thermal stability of the silver catalyst. If the level of Cl on the surface of the silver catalyst becomes too low, the reaction becomes excessively exothermic with accompanying loss of selectivity. Ethane acts as a chloride stripping agent and at concentrations above 5 mole percent and at temperatures typically employed in the epoxidation of ethylene, e.g., 240 to 280.degree. C., the degree of chloride stripping becomes unacceptably excessive. As is disclosed in the above-cited patents, one of the problems associated with the use of carbon dioxide as a heat transfer agent (heat sink) in ethylene epoxidation processes is that at levels greater than about 7 mole percent, the carbon dioxide becomes a reaction inhibitor for ethylene oxide formation. Thus, the concentration of carbon dioxide in feed gas in ethylene epoxidation processes must be limited to concentrations of less than about 7 mole percent. At such low levels carbon dioxide does not have an appreciable effect on the heat capacity nor the flammability characteristics of the gas mixture.
As explained in Lees, F. P., "Loss Prevention in the Process Industries, Volume 1," 485-86 (1980) and Coffee, R. D., Loss Prevention 13, 74-80, (1980), a flammable gas, e.g., methane, butane, and other alkane hydrocarbons, burns in oxidizing environments only over a limited composition range. The limits of flammability (often called the explosive or hot flame limits) are the concentration extremes at which a mixture of a flammable gas and an oxidant can continue to burn once a flame is ignited by an external energy source such as a spark. These flammability extremes are a function of temperature, pressure, and composition. The explosive limit is usually expressed as volume or mole percent flammable gas in a mixture of oxidant (usually oxygen), inert, and flammable gas. The smaller value is the lower (lean) limit and the larger value is the upper (rich) limit. For example methane-oxygen mixtures will propagate flames for methane concentrations between 5.1 and 61 mole percent methane and methane-air mixtures between 5.3 and 14 mole percent methane, at 25.degree. C. and atmospheric pressure. In general the lower explosive limit (LEL) decreases, and the upper explosive limit (UEL) increases as temperature and pressure increase, and amount of inert decreases.
Autoignition is defined as the spontaneous ignition of a vapor-air mixture as the result of the heat generated from exothermic oxidation reactions and in the absence of an external energy source such as a spark. The autoignition temperature (hereafter AIT) is the lowest temperature at which such an ignition will occur. Under these conditions the combustion of the flammable material generally goes to completion, i.e., carbon dioxide and water products. The combustion reactions can be very rapid and violent, i.e., explosive. The AIT is also a function of the composition, temperature, and pressure. A temperature rise of 800 to 200.degree. C. is expected with generation of a severe, destructive pressure pulse. Thus, if the reaction composition were in the flammable region and above the AIT, the mixture would spontaneously ignite or explode even without an initiating external energy source, with the ensuing destructive consequences to the reactor and potential hazardous vapor release.
At temperatures well below the autoignition temperature, oxidizable materials can also exhibit cool flame behavior, in which a partial oxidation mechanism prevails. Temperature rises of 10 to 200.degree. C. are common, without the severe pressure pulse associated with a hot flame. The transition between a cool and hot flame is ill-defined. In spite of the more mild consequences of the cool flame, this condition represents a potential safety concern, as it may lead to runaway epoxidation reaction and subsequent explosive behavior, as well as a loss of valuable product and diluent gases. Thus, from a safety and operational standpoint it is desirable to maintain the reactor composition, temperature, and pressure outside of both the cool and hot flame regions.
The use of diluent gases in non-allylic olefin epoxidation, specifically the epoxidation of 1,3-butadiene to 3,4-epoxy-1-butene, is described in U.S. Pat. Nos. 5,362,890, and 5,618,954. U.S. Pat. No. 5,618,954 teaches that nitrogen and C.sub.1 -C.sub.4 paraffinic hydrocarbons, especially methane, or mixtures therein are the preferred diluents for the epoxidation of 1,3-butadiene to 3,4-epoxy-1-butene The oxygen:butadiene ratio in the reactor feed gas can be increased by using methane as the diluent over that with nitrogen as the diluent without the methane:oxygen:butadiene mixture becoming flammable.
U.S. Pat. No. 5,362,890 discloses the use of C.sub.2 -C.sub.6 paraffin hydrocarbons as diluents for non-allylic olefin epoxidation. The data disclosed in this patent clearly teaches the advantages of using higher alkane hydrocarbons over methane, nitrogen and other common diluents. Advantages cited include higher safe oxygen levels, higher epoxide production levels for the same reactor temperatures, and more stable operation due to better heat removal. This patent specifically states that the use of branched-chain alkanes is not preferred over straight-chain paraffins due to the reactivity of tertiary hydrogen atoms of such branched-chain hydrocarbons with the surface chloride atoms of the silver catalyst. As with ethylene epoxidation, excessive stripping of chlorine from the surface of the silver catalyst tends to lower the selectivity of the epoxidation of non-allylic olefins.
U.S. Pat. No. 5,362,890 also teaches that straight-chain alkanes, e.g., normal butane, normal pentane, normal hexane, are preferred diluents, and that normal butane is the most preferred diluent for the epoxidation of 1,3-butadiene to 3,4-epoxy-1-butene. Although the use of n- butane increases the overall heat capacity of the reactor feed gas markedly over the heat capacity with methane or nitrogen as the diluent, at high oxygen concentrations, i.e., greater than about 20 mole percent, which are favorable for increased epoxide production, and with highly active catalysts, temperature gradients in a fixed bed reactor can be excessive, thereby resulting in unstable and unsafe operation. Branched-chain diluents are not preferred.
However, for straight-chain alkanes, as the number of carbon atoms in the diluent is increased from C.sub.4 Up to C.sub.5 and C.sub.6, e.g., n-pentane and n-hexane, the autoignition temperature of the diluent and the size of the cool flame region increases. The AIT drops in the series n-butane, n- pentane, n-hexane as 372.degree. C., 258.degree. C., 227.degree. C. respectively. In fact, at high concentrations of oxygen and at the higher end of the preferred range of reactor temperatures, the reactor composition can easily be in the cool flame or explosive flammable region and above the AIT. The diluent will begin to burn spontaneously, causing a rapid and uncontrollable rise in reactor temperature, i.e., a runaway reaction. Obviously a reactor cannot be operated safely in this regime. Thus, in order to maximize epoxide production it is important to use find other diluents which allow for efficient removal of heat and safe operation under high oxygen conditions.